The Democratic National Committee issued an apology on Monday to Bernie Sanders and his supporters over leaked emails that showed bias among party officials towards his opponent, Hillary Clinton, as the Democrats’ national convention opened amid chaotic scenes of Sanders supporters booing the nominee.

“On behalf of everyone at the DNC, we want to offer a deep and sincere apology to Senator Sanders, his supporters, and the entire Democratic Party for the inexcusable remarks made over email,” Democratic Party officials said in a statement. The controversy led to the resignation of Debbie Wasserman Schultz as party chair on Sunday.

“These comments do not reflect the values of the DNC or our steadfast commitment to neutrality during the nominating process,” the statement went on. “The DNC does not – and will not – tolerate disrespectful language exhibited toward our candidates.”

The statement further noted that the DNC was taking “appropriate action to ensure it never happens again”.

The FBI announced on Monday it was launching an investigation into the hack of the DNC’s email server that led to the publication of the emails, as Clinton’s bid for a week of unity began with renewed rancor among Sanders supporters who had long doubted the neutrality of national party officials.

Before events in the main arena in Philadelphia even began, the controversy over leaked internal emails had led to Wasserman Schultz being booed off stage by delegates from her home state of Florida.

Sanders supporters adopted one of Donald Trump’s attack lines against Clinton outside the hall, chanting “lock her up” at protests in downtown Philadelphia.

Sanders himself seemed incapable of controlling some of his more passionate supporters. Loud boos could be heard from within the closed-room meeting as the Vermont senator told them: “We have got to defeat Donald Trump and we have got to elect Hillary Clinton and Tim Kaine … This is the world we live in.”

The turmoil threatened efforts by Democrats to move on from the combative primary campaign during which Sanders ignited the progressive left and pushed Clinton all the way to the line.

Clinton will formally accept the Democratic nomination for president on Thursday, after set-piece speeches from Barack Obama, former president Bill Clinton and first lady Michelle Obama.

The party had arrived at the convention with hopes of drawing a stark contrast with Republicans, who officially nominated Trump at their own convention in Cleveland last week. But leaked emails suggesting top staffers at the DNC favored Clinton over Sanders, and allegedly looked for ways to undermine the Vermont senator’s campaign, cast a shadow on the pre-convention proceedings.

Wasserman Schultz faced a bitter reception while addressing Florida delegates at a breakfast on Monday, as a group of Sanders supporters drowned out the congresswoman with jeers.

“Shame on you!” some yelled, as others accused her of rigging the election in Clinton’s favor.

Wasserman Schultz, who announced on Sunday she will step down as DNC chair at the close of this week’s convention, tried at first to remain focused on the party’s agenda.

“We need to make sure we move together in a unified way,” she told the crowd.

But as the protests grew louder, Wasserman Schultz sought to tamp down the hecklers by acknowledging she had discussed her resignation with both Clinton and Obama.

“We know the voices in this room that are standing up and being disruptive, that’s not the Florida we know.”

“She reached out to Secretary Clinton to notify her about that and the secretary released a statement yesterday thanking her for her work.”

Officials with the campaign also expressed admiration for Wasserman Schultz’s decision to step down even though her term was not due to expire until January.

“For her to step down at the end of the week is really an extraordinary gesture on her part to achieve party unity and to achieve a distraction-free convention,” said Clinton spokesman Brian Fallon.

Mook sought to draw attention once more to claims that Russian hackers had leaked the emails in an effort to undermine Clinton and aid Trump.

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“What the experts said when this breach initially happened at the DNC is that they believed it was Russian state actors who took these emails,” Mook said.

“What further experts are saying is that then because they released these emails, that Russian state actors were feeding these emails to hackers for the purpose of helping Donald Trump.

“It’s troubling if it’s true,” he added.

Clinton spokeswoman Jennifer Palmieri also stressed the importance of the motivation for the security breach.

“These emails were hacked, reportedly hacked by the Russians, then [appeared] allegedly from WikiLeaks and Guccifer [the Romanian hacker], and they’re appearing right before our convention so it does appear that whoever is doing this is trying to influence our convention,” she said.

The FBI issued a statement on Monday to confirm it was investigating the breach and “working to determine the nature and scope of the matter”.

“A compromise of this nature is something we take very seriously, and the FBI will continue to investigate and hold accountable those who pose a threat in cyberspace,” the agency said in a statement.

Trump mocked the assertions at his campaign event, pivoting instead to the possibility that foreign governments might have access to Clinton’s private email server.

“She worked very, very hard to rig the system. Little did she know that China, Russia – one of our many, many ‘friends’ – came in and hacked the hell out of us,” Trump said.

“Why did Hillary get rid of her middle name? Hillary Rotten Clinton,” he added. (Clinton often goes by her full name, Hillary Rodham Clinton.)

Regardless of its origins, the DNC email leak has emboldened Sanders supporters, showing up in numbers with banners and T-shirts declaring “Bernie or Bust” and “Feel the Bern”. They have argued all along that the DNC tilted the scales in Clinton’s favor. Although they do not contain evidence that officials actively worked against the senator, top staffers exchanged emails that appeared to show personal bias toward Clinton and a desire to push narratives in the media that might hurt Sanders’ campaign.

The relationship between Sanders and the DNC was already fraught with hostility following a primary in which the senator’s campaign repeatedly tussled with party officials over the debate schedule, a joint fundraising agreement between Clinton and the DNC and access to the party’s voter database.

“The WikiLeaks emails reaffirm in the minds of delegates that we were given a raw deal and the primary was rigged from the very beginning to favor Mrs Clinton,” said Karen Bernal, a Sanders-supporting delegate from Sacramento.

“People will ask: ‘Why can’t we have party unity, we are here to pick a nominee,’ but Sanders owes his success to protest movements outside the party. So it should not come as a surprise that a lot of the expressions that we see are going to reflect the zeitgeist outside the convention.”

“We are a little pissed off,” added Manuel Zapata, another angry California delegate. “Since the moment we got here people have looked down on us when they walked past them with our campaign swag on.

“They throw party unity around as if it’s something that should make us jump for joy when her name is mentioned but everything that has happened over the last year pulls away from that.”

The email revelation, he added, “does not make us happy that we were right. This is heartbreaking for us.”

Sanders is scheduled to address the convention on Monday evening, as is Senator Elizabeth Warren.

Mook said he expected Sanders to “double down” on his support for Clinton and articulate the party’s shared commitment to a progressive agenda.

But Michael Lighty, the director of public policy at National Nurses United, a group that endorsed Sanders in the primary, said the senator’s speech “could get contentious”.

“It will not go down well [when he calls for unity],” Lighty said. “These delegates want to fight on the issues and they are not getting a chance.”

“Pleas for unity without movement on the issues, on policy, are not going to get us anywhere. You can’t paper it over.”